LYCOPODTNEAE.—HETEROSPOROrs LYCOPODIACEAE. 281 



formed by the coalescence of a number of vascular bundles (as polyarch), and this view 

 is supported by its resemblance to the axile cylinder of the roots. The leaves have each 

 a slender bundle of very simple construction, which runs very obliquely from the base of 

 the leaf through the cortex of the stem, and attaches itself lower down to an edge of the 

 xylem of the axile cylinder of the stem. 



The axile vascular strand is cauline, and may be followed up to close beneath the 

 apex, where it is a bundle of elongated cells (the initial bundle) ; the rows of spiral cells 

 of the xylem-bands are first formed, with which the similar formations in the leaf-bundles 

 become connected (Fig. 227) long before the tracheides begin to be developed. 



2. HETEROSPOROUS LYCOPODIACEAE'. 



The term heterosporous Lycopodiaceae may be used to designate the species of 

 Lepidodetidnvi which are characteristic of the coal-measures and which disappear in the 

 Permian formation. These were dichotomously branching tree-like plants, sometimes 

 thirty metres in height and thickly covered with lanceolate leaves which have left behind 

 them peculiar rhomboidal scars. These scars are made up of the 'leaf-cushion' and 

 the small leaf-scar properly so called. There is still much uncertainty with regard to 

 the structure of the stem. Renault states that a section of a branch of Lepidodendron 

 Rhadiaiinense shows a uniform central vascular cylinder consisting of tracheides with 

 transverse striation, which are broadest towards the middle of the cylinder. The leaf- 

 traces formed each of a single bundle unite with this cylinder, outside which is an endo- 

 dermis,and beyond this a strongly developed cortical tissue. The diameter of the stem in 

 this species was about five centimetres; the central vascular cylinder (' cylindre ligneux') 

 of the stem is hollow, owing perhaps to rupture of the enclosed tissue, and consists 

 of scalariform tracheides ; the leaf-traces are distinguishable at the periphery of the 

 cylinder. There is no certain indication of secondary growth in thickness, though the 

 condition of things may have been similar to that which will be described below in 

 Isoetes. The connection of fossil stems capable of great increase in thickness, such as 

 the Sigillarieae and Calavwdendrofi, with the Vascular Cryptogams is at present 

 questioned. The cortical tissue also is much more largely developed in the stem than 

 the vascular mass, and contains layers of sclerenchymatous fibres. The sporangiferous 

 spikes of Lepidodendron, of which we have fortunately some remains in a silicified state, 

 were on the ends of branches ; they are elliptical or rather elongated bodies known by 

 the name of Lepidosf?-obus, and are thickly covered with sporophylls, the lower portion 

 of each of which is set at right angles to the axis and bears a large niicrosporangium or 

 macrosporangium. The microsporangia are said to have been about two centimetres 

 long. Both kinds of sporangia were placed on the upper side of the base of the leaf. 

 The spherical macrospores were eight- tenths of a millimetre in diameter ; the length of 

 the chief axis in the tetrahedral microspore is given as one-tenth of a millimetre. The 

 two kinds of sporangia are on the same or on different spikes, but the latter condition 

 may be the result of the imperfect state in which we have them. 



> Renault, Cours de Bot. fossile, II Bd. The attempts to connect the Lepidodendreae with the 

 other heterosporous Lycopodineae (Selaginelleae and Isoeteae) have been less successful. These 

 plants have more resemblance to the homosporous Lycopodiaceae, with which we unite them ; 

 heterospory may very well have appeared several times in the same cycle of affinity. [Prof. William- 

 son contributes a note of his views on the structure and affinity of these forms to the second edition 

 of the English translation of Sachs' Lehrbuch; see also his Monographs in the Phil. Trans, from 

 187 1 to the present date.] 



